Semi-submersible vessels are known in the field of the art, and are widely used in the offshore industry, in particular for construction operations at sea, such as pipeline laying, pile driving, module installation, and other operations. Semi-submersible vessels are particularly used for the exploration, preparation and exploitation of oil and gas fields. The word ‘semi-submersible’ indicates that the vessel has a variable draft, which draft is controllable.
Offshore operations generally require a vessel that is stable in conditions with waves.
Prior to the advent of semi-submersible vessels, ship-shape vessels were used for offshore operations. The words ‘ship-shape’ indicate that this vessel has a ‘normal’ hull shape, in the sense that at an intersection plane where the hull intersects the water-line, the cross-sectional area of the hull, when viewed from above, is substantially the same as the surface area of the hull below the water-line. A ship-shape vessel advantageously has a relatively large allowable ratio of dead-weight to displacement at operational draft, which allows it to carry a substantial deckload and a substantial amount of equipment on deck.